AF Discussion

[brossart]

Can anyone point me to a good discussion on autofocus types, what are the

practical differences and which cameras have which ones?

 

I know that basically AF utilizes a horizontal string of pixels and measures

the contrast between pixels to determine appropriate focus. I understand that

cross type AF simply adds a vertical string of pixels as well, but what I'm not

sure of is does it use one or the other or both? Is cross type more accurate

simply because a horizontal stripe has the vulnerability of lining up with only

horizontal lines to focus on rendering the AF blind or is there more to it?

This question can be extended to the diaginal strings found in the center AF

point of the 40D.

 

Is the sensitivity or accuracy of AF better as you climb the line of cameras

(i.e. Rebel to 1D)? or is it essentially the same? Do camera's AF focus faster

or is it just dependant upon the abilities of the lens (e.g. faster motors)?

 

I know I appear to be focusing on a very narrow issue here, and yes, there is

much more to a camera that is important to me too, but I can find very little

discussion on this topic, and I am interested. One of my favorite things to

shoot are kids soccer games. The action is relatively fast, but being kids,

it's very hard to predict a certain spot on the field for maximum action and

thus to simply prefocus. I tend to simply follow the action up and down the

field, zooming in and out with the action. When my 'radar' goes off, I'm

already aligned on my subject so I just start shooting. The result is I'm

highly reliant on AF. Maybe too reliant?

 

One of the reasons I am so interested is that I am currently shooting a 350D

(with a slight front focus issue) and a 70-300/f4-5.6. I am considering

upgrading the lens to a 70-200/f2.8 to shorten my depth of field and to

eliminate the maddening seek that I get all too frequently with the 70-300.

I'm also considering upgrading to the 40D. While one option is to forego a lot

of the added functionality and simply get the forward focus issue fixed on the

350D, I have also heard that the 350D has a slightly inconsistant AF. I would

concur from my experience, but it could also be user error.

 

I appreciate your thoughts or direction on the matter. Thanks.

[picpocket]

Contrast based AF is just one way, and unfortunately a slower way

 

Most DSLRs use the phase detection mechanism to quickly figure out focus adjustment required and once determined simply ask the lens to adjust in a open loop arrangement (i.e. trust the lens, dont check again)

 

Some Details here:

 

AF: http://en.wikipedia.org/wiki/Autofocus

 

see the section on passive autofocus

 

and Canon methodology: http://mkoehler.de/index.php?option=com_content&task=view&id=15&Itemid=9

[mark u]

Here's a detailed explanation of how AF works in an SLR:

 

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

 

As you will see, phase detection based AF depends on two linear pixel arrays, each illuminated by a different part of the lens. Each linear array is most sensitive to contrast changes in a particular orientation - a cross sensor provides sensitivity regardless of subject orientation.

 

Actual AF performance speed depends on 1) the lens AF motor and the size of the elements moved and the distance they have to be moved; 2) the speed of calculation of the camera's AF circuit and the power delivered by the body to the lens; 3) the maximum aperture of the lens (a bigger aperture gives a clearer separation to work with and allows more accurate focus); a distant 4) is the amount of light so long as it is above a minimum - more important is that the AF sensor should be aimed at a clear contrast boundary, since AF will fail if there is no contrast to detect.

 

High end bodies do focus faster than lower end ones, but the lens characteristics are more important to overall focus speed. Another advantage is the closer packing of AF sensors, which reduces the risk of losing track on a moving subject.

[brossart]

Ashish & Mark, thanks for the info, it was very helpful. I'm a little surprised that Canon doesn't make this information more readily available. It strikes me as extremely relvent.

 

The components I found most useful were:

1. TTL-CT-SIR means Through The Lens-Cross Type-Secondary Image Registration. Understanding this, I see that even the Rebel XT has cross type AF.

2. Until the advent of the 40D, all dSLRs had cross type AF in the center, but not on the surrounding AF points. The 5D & 1D series may be different. I did not research these.

3. The acceptable focus range of non-cross type AF points is within the DOF of the max apperture of the lens, and the actual focal plane may vary from shot to shot as long as its within the DOF tollerance. This is huge considering that the cross type function is not activated for lenses slower than 2.8 (i.e. many consumer grade lenses).

4. The acceptable focus range for cross type AF points is 1/3 the DOF of the max apperature. So, using a 2.8 or faster lens actually benefits in two ways: by activating the cross type and by having a narrower DOF.

5. The actual size of the AF sensor is larger than the indicator in the view finder. Which means, a contrasty object close to the activated AF point that is on a different focal plane may throw off your focus.

 

Since the articles above were published prior to the 40D's diagonal cross type sensor, hyped by Canon as being even more sensitive, does anyone know if that means the focal tollerance is even smaller than the 1/3 DOF as set by previous cross type AF points?

[jimstrutz]

Not all cross type sensors are high precision (accurate to within 1/3 DOF). For instance all Digital Rebels have a center cross type sensor, but none are high precision. The 40D's AF points are all cross type, but only the center is high precision, and that is still "only" accurate to 1/3 DOF. When Canon says they are more sensitive, they probably referring to brightness and/or contrast, not precision.

 

"I know that basically AF utilizes a horizontal string of pixels and measures the contrast between pixels..." -- That's not really what it does, and that's a good thing.